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"He took it for two years, but the serious health problem he had in Iraq came back, so he is forced to leave a year early for genuinely serious health reasons," the official said on condition of not being identified. "He is sticking it out for a full year, which takes him to" a donors conference in Tokyo in July.

Nuland's statement said Crocker would step down "for health reasons in mid-summer, following the Kabul and Tokyo conferences."

White House Press Secretary Jay Carney hailed Crocker for having done "an extraordinary job in this current post, and he has been a key part of the implementation of the president's strategy in Afghanistan."

Carney said Crocker had come out of retirement at the request of President Barack Obama to take the ambassador's post and "the president's enormously grateful for that."

Nuland's statement cited a series of achievements during Crocker's tenure culminated by NATO's approval Monday of a timetable for the U.S.-led military mission in Afghanistan to end combat operations next year and withdraw by the end of 2014.

At the same time, NATO will launch a separate, new mission to train, advise and assist Afghan security forces starting in 2015.

Nuland's statement quoted Crocker as saying: "These achievements are the guarantee that as Afghanistan moves to a brighter future secured by its own capabilities, it does so ... in sovereign and equal partnership with the United States in particular and the international community generally."

No further details on Crocker's health problems or departure were immediately available.

This is not Crocker's first stint in the Afghan capital. After the Taliban were forced out of power, he was given the task in 2002 of reopening the U.S. Embassy in Kabul, according to his State Department biography.

Cameron Munter, the U.S. ambassador to Pakistan, is also due to leave his post this summer, as previously announced by the State Department.